1) Field of the Invention
The present invention relates to a method of producing a colored, precipitated calcium carbonate.
More particularly, the present invention relates to a method of producing a colored precipitated calcium carbonate containing a water-soluble dye firmly fixed to the precipitated calcium carbonate, and useful as a filler for producing a colored paper sheet having a strong color fastness without generating a difference in the hue and color depth of the front and back surfaces thereof.
2) Description of the Related Arts
In the production of paper sheets, for example, printing paper sheets, a pulp slurry is mixed with a relatively inexpensive filler, to impart various properties, for example, a desired surface smoothness, opacity and whiteness, and to lower the production cost of the paper sheets.
In a conventional acid paper-forming method, the filler comprises clay, talc or titanium dioxide. Most of these filler materials are imported, and thus are difficult to obtain and are expensive.
A recent increase in the demand for paper sheets having a high storage stability and durability has drawn attention to a neutral paper-forming method in which a paper sheet is formed at a neutral pH range. In this neutral paper forming method, a cheap natural limestone, which can be easily obtained from domestic sources, is used as a filler, i.e., a wet or dry pulverizing product of limestone is widely used as a ground calcium carbonate filler.
Also, precipitated calcium carbonate, which is produced by reacting an acid gas such as carbon dioxide gas with calcium hydroxide, is advantageous in that the whiteness and opacity thereof are higher, and the wire abrasion of a paper machine therewith is less than those of the ground calcium carbonate, and thus is now widely employed.
In view of an increased ease of reading, various colored paper sheets are utilized as printing paper sheets for books, in addition to white paper sheets.
In the conventional production of colored paper sheets, the pulp slurry is mixed with a filler and a dye. Usually, the dye usable for producing the colored paper sheets has a high dyeing effect on the pulp fibers, but the dyeing effect of the dye on the filler such as the light (precipitated) calcium carbonate is usually poor.
In a process for forming a paper sheet from a pulp slurry containing an additive, for example, a filler and dye, by using a single wire paper machine, a resultant wet paper layer formed on a wire net is forcibly dehydrated through the wire net while allowing portions of the additive and fine pulp fibers to flow out through the wire net, and therefore, the distribution of the additive in the wet paper layer changes along the Z direction of the paper layer (at a right angle to the wet paper layer surface). Namely, usually the amount of the additive distributed on the front surface portion of the paper layer is larger than that on the back surface portion thereof.
Therefore, when the pulp slurry contains a non-colored filler and a dye, although the dye is uniformly absorbed by the pulp fibers during the paper-forming process, if the non-colored filler is unevenly distributed in the colored pulp paper layer, as mentioned above, a difference in the color of the resultant paper sheet occurs at the front and back surfaces thereof.
If the filler is made the same color as the colored pulp fibers, even if the colored filler is unevenly distributed, no difference appears in the color of the front and back surfaces of the resultant paper sheet.
The precipitated calcium carbonate, however, has a poor affinity to the dye, and thus, even when dyed with the dye, when the dyed precipitated calcium carbonate particles are brought into contact with pulp fibers, the dye on the precipitated calcium carbonate particles is moved therefrom to the pulp fibers and the dyed precipitated calcium carbonate particles are decolored, and thus, a color difference appears between the front and back surfaces of the resultant colored paper sheet.
Various attempts have been made to eliminate the above-mentioned color difference between the front and back surfaces of the colored paper sheet.
For example, Japanese Examined Patent Publication No. 61-70099 discloses a method of producing a colored paper sheet, in which method a pigment and a dye having the same color as each other are concurrently employed. This method, however, is disadvantageous in that the pigment is expensive in comparison with the dye, and thus the cost of the resultant colored paper sheet is undesirably raised. Also, this method causes the content of the pigment, which is a water-insoluble substance in the pulp slurry, to be increased, and thus the inside of the paper-forming machine is stained by the water-insoluble substance. Further, this method is disadvantageous in that an additional device for supplying the pigment to the pulp slurry must be added to the paper-forming machine, and thus the paper-forming operation becomes complicated. Furthermore, when an organic or inorganic pigment containing a heavy metal is employed, there is a risk of environmental pollution by the heavy metal, and thus the use of the pigment is not permissible.
In another method, a pulp slurry is mixed with a filler and a dye, to concurrently color the pulp fibers and the filler with the dye, and the resultant dye-containing pulp and filler slurry is used to produce a colored paper sheet.
In still another method, a filler is first dyed with a water-soluble dye, and then a pulp slurry mixed with the colored filler is subjected to the paper-forming procedure.
Nevertheless, the colored calcium carbonate produced by the conventional method has a poor color fastness, and thus the above-mentioned conventional methods are not successful in practice.
Accordingly, there is a great demand for the provision of a colored precipitated calcium carbonate having a strong color fastness.